It is standard as described in U.S. Pat. No. 6,036,157 of C. Baroin to mount a motor-vehicle seat on a pair of rail assemblies each including a fixed rail secured to the vehicle frame, a movable rail fixed to the seat, and a plurality of roller elements engaged between the rails and supporting the movable rail for longitudinal sliding in the fixed rail. A cage or holder can be provided to maintain the roller elements—balls or cylinders—at a uniform spacing. Such a system is fairly complex, but allows the seat position to be adjusted easily.
A principal disadvantage of this known arrangement is that the load is transferred between the rails at very limited locations, at points in a ball-type system. Thus when there is a great deal of stress, as for instance in an accident when the seat's inertia is suddenly applied transversely via the movable rail to the fixed rail, the result is deformation or da age to the rails. In fact in such a system the rails can separate. Another disadvantage of the known systems is that the numerous parts are expensive to manufacture and, because of their complexity, prone to failure.